A device of this type is already known, e.g. as shown in FIG. 7, that comprises an axial bearing 120 comprising a stator 122 of ferromagnetic material presenting an annular housing concentric with the axis of the rotor 10 for receiving a coil 123. The active surfaces 125, 126 of the stator 122 situated on either side of the housing in which the coil 123 is placed co-operate with a disk-shaped armature 11 that is secured to the rotor 10 and that is essentially perpendicular to the axis of the rotor 10. A sensor 35, which may be of the inductive, optical, or capacitive type, is also associated with the axial bearing 120 to detect the position of the rotor armature 11 relative to the stator 122 and deliver a signal to servo-control circuits (not shown) that power the coil 123 in order to create a magnetic field such that the active surfaces 125, 126 of the stator 122 can exert a force of attraction on the armature 11 so as to maintain it in an axial position that is stable. An axial bearing of the same type may also be disposed symmetrically relative to the armature 11 so as to exert a force of attraction on the second plane face of the armature 11 that is perpendicular to the axis of the rotor 10.
The structure 20 on which the stator 122 is mounted may also serve as a support for a radial magnetic bearing 30 disposed close to the axial bearing 120.
The radial active magnetic bearing 30 may comprise a stator 31 of laminated ferromagnetic material which is mounted on the structure 20 and includes electromagnetic windings 32 connected by connection wires 33 to power-supply and servo-control circuits (not shown). The radial magnetic bearing 30 further comprises an annular armature 34 likewise made of laminated magnetic material that is fitted on the rotor 10 and is thus concentric with the rotor 10. A detector 35 detects the radial position of the rotor 10 and may be placed on a support secured to the structure 20 in the vicinity of the radial bearing 30 in order to detect the radial position of the reference surface 36 at the periphery of the rotor 10 that faces the detector 35. The signals from the detector 35, which may be of the inductive, capacitive, or optical type, are applied to circuits for servo-controlling the current supplied to the electromagnet windings 32. In the example of FIG. 7, the detector 35, which is of the inductive type, serves to detect the position of the rotor 10 both in an axial direction and in two mutually perpendicular radial directions. The reference magnetic surface 36 is sandwiched in the axial direction between two surfaces of non-magnetic material.
In the device of FIG. 7, which uses only one coil 123 within the axial magnetic bearing 120, for a rotor armature 11 of given outside diameter, a maximum load-bearing surface area is obtained between the active surfaces 125 and 126 situated on either side of the housing for the coil 123 and the plane face of the rotor armature 11 which is situated facing these active faces.
However, the coil 123 creates circuits 101, 102 of non-zero magnetic flux circulation through the stators of the radial magnetic bearing 30 and of the position detector 35, through the rotor armatures 34, 36, 11, and through the shaft 10.
More particularly, the circuit 101 leads to the radial bearing 30 being magnetized, thereby leading to a loss in its capacity and creating coupling between the radial force and the axial force.
The circuit 102 leads to magnetization of the position sensors, leading to a loss of sensitivity thereof and creating coupling between measurements and the axial force.
The device shown in FIG. 7 thus presents the major drawback of creating significant amounts of magnetic leakage.
In order to remedy that problem and avoid magnetizing the surroundings of the magnetic abutment constituting the axial bearing, the solution shown in FIG. 8 has been proposed, in which figure those elements of the rotor 10, of the structure 20, and of the radial magnetic bearing 30 that are unchanged carry the same references and are not described again.
In the solution proposed with the device of FIG. 8, the axial magnetic bearing 220 has a stator 222 with two annular housings concentric with the axis of the rotor 10 for receiving coils 223 and 228.
By using an even number of coils 223, 228 and by causing current to flow in the coil 228 in the opposite direction to the current flowing in the coil 223, it is possible to ensure that each closed outline 201, 202 surrounding the coils 223, 228 perceives magnetic excitation that is zero.
The solution shown in FIG. 8 thus makes it possible to avoid the surroundings of the axial bearing 220 being magnetized by the magnetic excitation created by the coils of said axial bearing. This avoids magnetization interfering with the radial bearing 30, with the position sensor 35, or with the entire surroundings of the axial bearing.
Nevertheless, the fact of using two coils 223, 228 situated in two open housings reduces the active surface areas 225, 226, 227 that co-operate with the armature 11, thereby leading to a loss of load-carrying area for a disk-shaped armature 11 of given diameter. Unfortunately, in various applications, given the high speed of rotation of the rotor, it is not possible to increase the diameter of the rotor armature in the axial bearing beyond certain limits, so that implementing multiple coils within an axial bearing becomes problematic because of the residual active surface areas no longer being large enough, thereby limiting the capacity of the axial bearing.